Sunday, December 14, 2008

from A Philosophical and Mathematical Dictionary, 1815

(page 68)

As the moon has on her surface mountains and valleys in common with the earth, some modern astronomers have discovered a still greater similarity, viz, that some of these are really volcanoes, emitting fire, as those on the earth do. An appearance of this kind was discovered some few years ago by Don Ulloa in an eclipse of the sun. It was a small bright spot like a star near the margin of the moon, and which he at that time supposed to be a hole or valley with the sun's light shining through it. Succeeding observations, however, have induced astronomers to attribute appearances of this kind to the eruption of volcanic fire; and Dr. Herschel has particularly observed several eruptions of the lunar volcanoes, the last of which he gives an account of in the Philos. Trans. for 1787, April 19, 10h. 6m. sidereal time.

"I perceived," says he, "three volcanoes in different places of the dark part of the new moon. Two of them are either already nearly extinct, or otherwise in a state of going to break out; which perhaps may be decided next lunation. The third shows an actual eruption of fire or luminous matter: its light is much brighter than the nucleus of the comet which M. Mechain discovered at Paris the 10th of this month." 

The following night he found it burnt with greater violence; and by measurement he found that the shining or burning matter must be more than 3 miles in diameter; being of an irregular round figure, and very sharply defined on the edges. The other two volcanoes resembled large faint nebulae, that are gradually much brighter in the middle; but no well-defined luminous spot was discovered in them. He adds, "the appearance of what I have called the actual fire, or eruption of a volcano, exactly resembled a small piece of burning charcoal when it is covered by a very thin coat of white ashes, which frequently adhere to it when it has been some time ignited; and it had a degree of brightness about as strong as that with which a coal would be seen to glow in faint day-light. 

In a letter by M. Lalande, it is said that, the 13th inst. from 7 to 9 in the evening, Dom. Nouet, one of the astronomers of the Royal Observatory, perceived, in the unenlightened part of the moon, what Dr. Herschel has called a volcano, like a star of the sixth magnitude, or one of the cloudy ones, the brightness of which increased from time to time, as if by flashes. Other astronomers have perceived it, and M. de Villeneuve had seen it before, on the 22d of May, 1787. We cannot therefore doubt of the existence of this volcano in the moon. Dr. Herschel saw it the 4th of May, 1783, and particularly the 19th of April, 1787. In the eclipse of the 24th of June, 1778, M. d'Ulloa, a well-known Spanish astronomer, had seen on the dark disc of the moon, a bright point; and in the total eclipse of 1715, certain curious observers had perceived some flashes of light. 

There is no sensible atmosphere in the moon, it is true, and chemists may dispute about the name of volcanoes being given to such apparent eruption; but the name after all is of no consequence, and we must certainly subscribe to Dr. Herschel's opinion. This volcano is situated on the north-east part of the moon, about three minutes from the moon's border, towards the spot called Helicon, marked No. 12 in the figure of the moon in Lalande's astronomy. On the next day, March the 14th, Jupiter had been eclipsed by the moon. This rare and curious phenomena has been observed by all astronomers. 

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